1. Field of the Invention
The present invention relates to apparatus for controlling the pulse period of pulses applied to a pulse motor. More particularly, the invention relates to apparatus for driving a pulse motor used in a printer of the type in which the printing head is moved by a pulse motor in the direction across a printing paper to effect printing.
2. Description of the Prior Art
In a wire printer or a thermal printer, the printing head is moved in the direction across a printing paper to effect printing. To make it possible to carry out intermittent printing with such type of printer there is often used a pulse motor as means for driving the printing head. This is because the use of pulse motor brings forth the following advantages:
(1) Since it is driven in response to a pulse signal, instantaneous start and instantaneous stop are possible.
(2) At start and stop there is generated no noise.
(3) It has a long useful life and high reliability.
In the case of a DC motor or AC motor, the rotational speed of the motor gradually increases up and it can reaches a constant speed only after a transition stage of rotation. On the contrary, a pulse motor can start rotating at a uniform rotational speed at once when the first pulse is applied to the motor. There is no transition stage or, if any, it is extremely short. This is another advantage of pulse motor. However, the use of a pulse motor in a printer involves some problems. There is a limitation regarding the first pulse applied to the pulse motor. Because of the inertia force on the rotor and driving mechanism, the pulse period of the first pulse can be reduced only to a certain limit value. This is disadvantageous to high speed driving of printing head. In a printer, it is often required to drive the printing head at a higher speed. For example, this is applied to the case of carriage return. In this case, according to the prior art, the necessary high speed driving is attained by reducing the pulse period of the second and subsequent pulses relative to the pulse period of the first pulse. This prior art technique will be described in detail hereinafter with reference to FIG. 1.
FIG. 1 shows characteristic curves of a common pulse motor generally used in printers.
Curve 1 is a torque/frequency relation curve for self-start and curve 2 is a torque frequency for continuous response. From FIG. 1, it will be understood that when 50 g.cm torque is required to drive a carriage, the pulse motor should be started with the frequency of 500 pps (pulse period: 2 millisecond) which is indicated by the intersection 6 with the curve 1. However, if the pulse motor is continuously driven with the frequency of 500 pps, then it operates in a mode of continuous response and therefore the relation between torque and frequency thereof is shifted from curve 1 to curve 2. As seen from curve 2, in this case, there is produced about 115 g.cm of torque which is more than sufficient for this purpose. Therefore, instead of keeping the frequency at 500 pps, the frequency is gradually increased up from 500 pps to 1000 pps (pulse period: 1 m.s.) and then it is kept constant at 1000 pps. By doing so, the relation between torque and frequency is shifted from the intersection 6 on curve 1 to the point 7 at which curve 2 and 1000 pps line intersect. Thus, it will be understood that the motor speed can be increased up to a value two times larger than at the start time while obtaining the constant torque of 50 g.cm. In summary, FIG. 1 indicates that 600 pps is the maximum self-start frequency of the pulse motor and with any frequency larger than 600 pps the pulse motor can not start, and that by gradually increasing the frequency starting with 500 pps it is made possible to drive the pulse motor at a higher speed up to 1000 pps while obtaining the constant output of 50 g.cm at the same time.
In accordance with the above teachings, the pulse motor used in the printer of the prior art has been driven to attain a rapid carriage return. That is, to drive the printing head at a higher speed, the pulse motor is started with the first pulse having a pulse period larger than the determined value and then the pulse period of pulses subsequent to the first pulse is gradually reduced. However, it has been found that this prior art technique has the following problems:
(1) The pulse period is controlled in such manner that the first pulse has a pulse period of 2 m.s. and after reaching the constant speed, pulses have a pulse period of 1 m.s. In this case, it is required to reduce the pulse period to that of the constant pulse stepwise through at least several pulses.
(2) Overrun of carriage often occurs when the carriage running at a high speed is stopped by an abrupt stoppage of pulse. To prevent such trouble, a stopper has been provided conventionally in a printer. However, noise is generated by impact of the carriage against the stopper. In the worst case, the carriage is damaged.